The present invention relates to an improved two-stage pump assembly for use in a liquid chromatograph or other instrument.
Some instruments must deliver fluid, such as liquid and liquefied gas, at a constant flow rate. For example, a liquid chromatograph uses a pump that delivers fluid to a column or introduces a sample at a constant flow rate. Double plunger pumps of the structure as shown in FIG. 1 are often employed for such purposes. For the operation of the double plunger pump, see U.S. Pat. No. 4,436,230.
Referring to FIG. 1, a pump assembly is mounted on a support 1. Plunger pumps 2 and 3 have cylinders 4 and 5, respectively, which are mounted to the support 1. Plungers 6 and 7 are fitted in the cylinders 4 and 5, respectively. As the plungers reciprocate, the volumes of the chambers 8 and 9 of the cylinders are changed. Check valves 10 and 11 are mounted at the suction port and the discharge port, respectively, of the chamber 8, and check valves 12 and 13 are mounted at the suction port and the discharge port, respectively, of the chamber 9, to allow fluid to pass only in the direction indicated by the arrows. A pipe 17 is connected to the valves 11 and 13 mounted at the suction ports, and is immersed in a fluid (sample) 14 contained in a reservoir 15, the fluid 14 being under the atmospheric pressure. A pipe 18 which is connected to the valves 10 and 12 mounted at the suction ports is coupled to the separation column of a liquid chromatograph 16. An electric motor 19 is mounted on the support 1 and has its shaft 20 fixed to cams 21 and 22 that allow the plungers 6 and 7 to be moved at constant velocity as described later. Rotors 23 and 24 attached to ends of the plungers 6 and 7, respectively, are always pressed against the sides of the cams 21 and 22, respectively, by the resilient force of springs 25 and 26, respectively. The spring 25 is inserted between the jaw of the plunger 6 and the cylinder 4. Similarly, the spring 26 is inserted between the jaw of the plunger 7 and the cylinder 5.
FIGS. 2(a) and 2(b) show the cams 21 and 22, respectively, installed in the assembly shown in FIG. 1 as viewed from the direction of the axis of the shaft 20, which is taken as Y axis. The cams 21 and 22 are identically shaped into the form of a heart, and are symmetrical with respect to an axis, for example Z axis, perpendicular to the Y axis. The cams 21 and 22 mounted to the shaft 20 are spaced 180.degree. from each other. The rotors 23 and 24 of the plungers bear on the sides of the cams 21 and 22, respectively, at points 27 and 28, respectively. As the motor shaft 20 is rotated, the points 27 and 28 move right and left on the X axis. When the shaft 20 is rotated to the right as indicated by the arrow in FIG. 2 by the motor 19, the plungers 6 and 7 are moved in opposite directions along the X axis. That is, the plunger 6 moves to the right, while the plunger 7 shifts to the left as viewed in FIG. 1. The cam profile of the heart-shaped cams 21 and 22 is so made that the plungers move at constant velocity as long as the rotational velocity of the motor 19 is constant. In this way, while the plunger 6 or 7 travels to the right, the volume of the chamber 8 or 9 increases, closing the valve 10 or 12 at the discharge port. At the same time, the valve 11 or 13 is opened at the suction port. As a result, the sample 14 is drawn into the pump. On the other hand, while the plunger moves to the left, the volume of the chamber 8 or 9 decreases, closing the valve 11 or 13 at the suction port. Simultaneously, the valve 10 or 12 at the discharge port is opened. Thus, the fluid is delivered to the separation column of a liquid chromatograph 16 from the pump chamber.
Since the plungers 6 and 7 move in 180.degree. out-of-phase relationship as mentioned previously, the chamber 8 first takes in the liquid and, at the same time, the chamber 9 pumps the liquid out of it under the condition shown in FIG. 2. Subsequently, the chamber 8 delivers and the chamber 9 fills at the same time. These operations are illustrated in FIG. 3, where time or the angular position of the cam 21 or 22 is plotted on the horizontal axis and the displacements of the plungers 6 and 7 on the vertical axis. The displacement of each plunger is defined as the distance between the point of the plunger 27 or 28 at which it is in contact with the roller and the fundamental circle 29 (FIG. 2) of the cam. The displacement of the plunger 6 is indicated by the solid line, whereas the displacement of the plunger 7 is denoted by the broken line. SA1 and SA3 indicate suction strokes of the chamber 8; SB1 and SB3 indicate displacement strokes of the chamber 9; SA2 and SA4 indicate displacement strokes of the chamber 8; and SB2 and SB4 indicate suction strokes of the chamber 9. If the rotational velocity of the motor is increased to increase the translating velocity of the plunger, then the gradients of the straight lines shown in FIG. 3 become steeper, increasing the amount of fluid displaced per unit time. Usually, this amount is simply known as flow rate, which is proportional to the pressure at the discharge port of the pump.
Referring next to FIG. 4, there are shown changes in the flow rate of the pump assembly shown in FIG. 1 with time. The solid lines represent the flow rate due to the displacement stroke of the first pump chamber. The broken lines indicate the flow rate attributed to the displacement stroke of the second pump chamber. If the flow rate were completely constant, then these solid and broken lines should be parallel to the horizontal axis, or time. Usually, however, the flow line experiences flow resistance, and therefore the delivery pressure is higher than the atmospheric pressure. Immediately after the pump is switched from one filling stroke to its next stroke of displacement, the pressure inside the pump chamber is equal to the atmospheric pressure, so the valve at the discharge port does not open and the liquid is not delivered during a short time that ends when the pressure inside the pump reaches the higher delivery pressure. In this way, the plunger compresses the liquid while the pressure inside the pump chamber varies from the atmospheric pressure to the delivery pressure. This compressing action is absorbed by the compression of the liquid itself and by deformation of the materials sealing the pump chamber. This absorption results in the flow rate of the pumps 2 and 4 to drop, as indicated by 32 in FIG. 4.
Where the analytical column of a liquid chromatograph is connected to the discharge port, if the flow rate of the pump varies or drops as shown in FIG. 4, the base line of the obtained spectrum will pulsate, making accurate measurement impossible. This phenomenon becomes conspicuous especially where a load having a large fluid resistance is connected to the discharge port of the pump, or where a fluid having a large compressibility, such as liquefied gas, is used. Accordingly, the actual average flow rate including flow rate pulsations is lower than the average flow rate that is obtained by calculation without taking into account the aforementioned absorption of the compression action. This makes it very difficult to accurately control the flow rate of the pump to a desired value. A conventional solution to this difficulty is to forecast the effect of the absorption of the compression action and to remove the effect. This scheme is realized by a system as shown in FIG. 5, where the conventional double plunger pump already described in connection with FIG. 1 is indicated by numeral 33. Liquid sample 14 contained in reservoir 15 is drawn into the suction port of the pump 33 via a pipe. A flow resistor 34 is connected to the discharge port of the pump 33. A pipe extends from the exit of the resistor 34 into the atmosphere. A measuring vessel 35 is placed below this pipe. A means 36 for measuring pressure is installed between the discharge port of the pump 33 and the flow resistor 34. In the operation of this system, the pump 33 is driven at a constant rate. The resistance value of the flow resistor 34 is varied successively from zero to a high value. At each resistance value the pressure and the flow rate are measured. The calibration curve shown in FIG. 6 was derived in this way. The flow rate is determined by measuring the volume of the liquid sample 14 stored in the vessel 35 per unit time. The calibration curve indicates that when the pressure at the discharge port of the pump is P.sub.0, the actual flow velocity is 0.8 times as high as the flow velocity obtained when the discharge port of the pump is under the atmospheric pressure. Therefore, the decrease in the flow rate can be compensated by increasing the rotational velocity of the motor by a factor of 1/0.8. However, this method introduces the following problems: (1) The calibration curve varies among fluids to be delivered; (2) The calibration curve described above must be prepared for each individual pump; (3) Since the volume of pump absorbed as described above depends on the performance of the components of the pump, the calibration curve varies with time.
One means to moderate the decrease in the average flow rate due to the higher delivery pressure is to lengthen the stroke of the plunger, which increases the maximum volume Q of the pump chamber. Thus, for a certain flow rate the period of movement of the plunger is extended. In other words, the period of the reciprocating motion increases. As a result, the frequency of absorptions of the compressing action per unit time is reduced. Also, since the volume q absorbed as mentioned above is maintained substantially constant irrespective of the maximum volume Q of the pump chamber, the decrease in the average flow rate due to the higher delivery pressure can be held down to a low value. However, if the period of the displacement of plunger, or the period of the occurrence of flow rate pulsations, is increased to an extremely large value, say 1 minute, then normal function of the load on the pump to smooth out flow rate pulsations or the function of a damper for removing pulsations will become inoperative. A further consideration is that, in general, as the period of the reciprocating motion of a pump increases, it becomes impossible or more difficult to continuously change the flow rate of the pump.
Recently, it has been often required that pumps for use in analytical instruments deliver fluid at quite small flow rates. The minimum controllable flow rates needed for normally adopted analytical regions are as follows:
(I) microanalysis--0.1 .mu.l/min. PA1 (II) semimicroanalysis--1 .mu.l/min. PA1 (III) macroanalysis--10 .mu.l/min.
Unfortunately, it is quite difficult to cover these three ranges of flow rates by the use of the pump assembly shown in FIG. 1, where the cams are rotated in one direction to drive the plungers, because if the cams were designed to accommodate any one of the flow rate ranges, then the rotational velocity of the stepper motor for rotating the cams would have to be set to such a high value that the plungers cannot easily follow them; or inversely the velocity of the stepper motor would have to be so slowed down that it can no longer rotate smoothly.